SYNOPSIS

DESCRIPTION

Reads a portable graymap as input. Produces a portable bitmap as
output.
Note that there is no pbmtopgm converter. Any program that uses the
Netpbm libraries to read PGM files, including virtually all programs in
the Netpbm package, will read a PBM file automatically as if it were a
PGM file.
If you are using a less intelligent program that expects PGM input, use
pnmdepth to convert the PBM file to PGM. As long as the depth is
greater than 1, pnmdepth will generate PGM. This less intelligent
program quite probably is also not intelligent enough to deal with
general maxvals, so you should specify a depth of 255.

OPTIONS

The default quantization method is boustrophedonic Floyd-Steinberg
error diffusion (-floyd or -fs). Also available are simple
thresholding (-threshold); Bayer's ordered dither (-dither8) with a
16x16 matrix; and three different sizes of 45-degree clustered-dot
dither (-cluster3, -cluster4, -cluster8). A space filling curve
halftoning method using the Hilbert curve is also available.
(-hilbert);
Floyd-Steinberg will almost always give the best looking results;
however, looking good is not always what you want. For instance,
thresholding can be used in a pipeline with the pnmconvol tool, for
tasks like edge and peak detection. And clustered-dot dithering gives
a newspaper-ish look, a useful special effect.
The -value flag alters the thresholding value for Floyd-Steinberg and
simple thresholding. It should be a real number between 0 and 1.
Above 0.5 means darker images; below 0.5 means lighter.
The Hilbert curve method is useful for processing images before display
on devices that do not render individual pixels distinctly (like laser
printers). This dithering method can give better results than the
dithering usually done by the laser printers themselves. The -clump
flag alters the number of pixels in a clump. This is usually an integer
between 2 and 100 (default 5). Smaller clump sizes smear the image less
and are less grainy, but seem to loose some grey scale linearity.
Typically a PGM image will have to be scaled to fit on a laser printer
page (2400 x 3000 pixels for an A4 300 dpi page), and then dithered to
a PBM image before being converted to a postscript file. A printing
pipeline might look something like: pnmscale -xysize 2400 3000
image.pgm | pgmtopbm -hil | pnmtops -scale 0.25 > image.ps
All flags can be abbreviated to their shortest unique prefix.

REFERENCES

The only reference you need for this stuff is "Digital Halftoning" by
Robert Ulichney, MIT Press, ISBN 0-262-21009-6.
The Hilbert curve space filling method is taken from "Digital
Halftoning with Space Filling Curves" by Luiz Velho, Computer Graphics
Volume 25, Number 4, proceedings of SIGRAPH '91, page 81. ISBN
0-89791-436-8